Well, the consensus seems to be that the pattern leader must be coincident with the origin of the curve. So in my case, that would mean redefining the curve to divide the top segment and place the origin where my first hole is. That seems to make sense, although it's a frustrating limitation and the results are unexpected. I didn't test it because after sending this I decided that even hole spacing wouldn't fit my application after all so the pattern didn't make sense anymore.
Several folks suggested that I needed to first create a datum point on the curve, pattern the point then create my sketch anchored to only that point and pattern the cut as a reference pattern to the point pattern. I did try that, but the point patterns just like the cut - shifted to the right by the same offset as the pattern leader.